Since the discovery of electro-magnetic radiation, there has been an almost continual effort to devise oscillators that will work at ever higher frequencies. If the frequency is greater than 0.1 GHz, it is commonly termed "microwave." One type of microwave oscillator uses microwave tubes which are capable of operating at frequencies as high as 300 GHz although lower frequencies are more typical. These tubes are typically complex metallic structures which use an appropriately modulated electron beam coupled to the desired electro-magnetic mode to permit energy transfer into the mode. Microwave tubes use a nonresonant circuit structure having the form of re-entrant cavities, helical slow wave structures, or coupled cavities as well as other structures. Even today, it is still probably true that microwave tubes furnish the highest power levels at most frequencies.
However, because of the desire to reduce both the size and cost of the oscillators, there has been a search for alternative structures which has recently, that is, for the last several decades, focused primarily on solid state, and especially semiconductor, oscillators. There is also a desire to make such oscillators which are compatible with integrated circuit (IC) devices. Several types of devices, including those using transferred electron effects, such as the Gunn diode, as well as those using transit time effects, such as the IMPATT diode, are exemplary of this art.
These semiconductor devices typically employ a semiconducting diode as a negative conductance element in an oscillator circuit including a tuned cavity to achieve frequency stability, tunability, efficiency and noise reduction. The negative conductance device, which serves as a power source, is placed in parallel with the tuned cavity, that is, the positive conductance element, thereby permitting the generation of high frequency oscillations which are characterized by the cavity mode. However, it will be readily appreciated that tunability is greatly restricted.
There are other device limitations and operating characteristics which appear to be intrinsic to such device structures. For example, during each cycle, a single charge packet is formed near one end of the diode and then traverses a portion of the drift region before being either dispersed or absorbed. Higher frequencies are generally sought by making the devices either or both smaller and capable of being driven still harder. It will be appreciated that ultimately the response times of the carrier distribution will limit the obtainable frequencies. Additionally, the device can support only one charge packet at any time. This limitation arises because the space-charge field due to the packet produces a voltage drop sufficient to drive the electric field in the remainder of the device below the threshold for charge-packet formation until the existing packet is dispersed. This, of course, also limits the maximum attainable frequency.
Except for high frequency transistors which are three terminal devices, the semiconductor oscillators, are generally bulk, two terminal structures. High frequency transistors are often limited in their maximum operating frequency by both channel length and the resistive and capacitive parasitics. The two terminal structures use field distributions which are modified and controlled by doping profiles while power is dissipated throughout the device. If the device is sufficiently thin, the electrical contacts may also contribute to controlling the device operation. These features reduce the available design alternatives. Of course, microwave amplifiers are also of considerable interest.